1. Field of the Invention
The present invention relates to the field of cold forming tubular materials and in particular to an apparatus and method for forming and piercing a complex-shaped tubular frame member from a tubular blank.
2. Description of the Prior Art
The principal frame design for automobile frame members is of the "box" type construction for strength and load bearing purposes. These frame members often have great variation in both the horizontal and the vertical profile. The cross-section of such tube members also often varies rather extremely from approximately a square cross-section, to a rectangular cross-section to a round cross-section to a severely flattened cross-section, and to any irregularly shaped combination of the above. While some simple, large radiused profiles with varying cross-sections have been obtained by cold or heat forming a generally cylindrical tube blank, most current manufacturing methods produce the complex-shaped box section tube member by fabricating two "U" section stampings which are then welded together to form the finished part. Unfortunately, material and labor consumption in these processes is enormously inefficient.
The general operations of bending, stretching, depressing and radially expanding a tube blank, with or without a mandrel, are known. For the majority of metals, it is fairly easy to bend small diameter tubing into an arc having a large radius. But as the diameter of the tubing increases and the radius about which it is to be bent decreases, the tube bending process requires some combination of compression at the inner bending radius of the tube and stretching at the outer radius. Although the outer bending surface of the tube may be stretched to the full extent of the materials rated elongation characteristics, one cannot satisfactorily bend a tube with a given diameter about a relatively small bending radius without encountering severe buckling at the inner bending surface or undesirable deformation at the outer bending radius. Some have achieved bending tubes with a certain diameter about relatively small bending radii by controllably dimpling or allowing controlled rippling of the inner tube surface thereby creating less stretching of the outer tube surface.
Other examples of methods for bending a tube are disclosed in U.S. Pat. No. 4,704,886, which shows internally pressurizing a tube blank, gripping the opposite ends of the blank and applying longitudinal tension at the ends while applying a lateral force against the blank to bend the blank. U.S. Pat. No. 4,567,543, discloses depressing regions of the tube blank and then expanding the blank within a complementary shaped cavity formed by a pair of dies. U.S. Pat. No. 4,829,803 discloses forming a box-like frame member by internally-pressurizing a preformed tubular blank, closing a pair of die halves around the blank to partially deform the blank within mating die cavities, and then increasing the internal pressure to exceed the yield limit of the wall of the blank to expand the blank into conformity within the mating die cavities.
In addition, holes have to be cut or pierced into each box section tube member in order to join the members together to form a space frame for an automobile. To pierce or cut the holes into the tube member a solid mandrel, die block or die button is inserted into the interior of the tube and placed against the surface of the tube to be pierced. A punch then strikes the exterior surface of the tube to cut the hole. The mandrel, die block or die button is used to support the wall or surface of the tube as it is being pierced. If the wall or surface is not supported, a hole can not be neatly pierced in the tube. Instead the surface of the tube surrounding the hole has a tendency to become deformed. In order to place the mandrel, die block or die button against the interior surface of the tube, it is easier to pierce holes in the two "U" section stampings before assembling and welding the two sections together.
It is therefore desirable to provide an apparatus to form a tubular blank into a frame member having variations in the vertical and horizontal profiles and in the cross-sectional configuration while reducing the amount of variation in the wall thickness of the finished frame member.
It is also desirable to provide an apparatus for piercing and cutting holes in a finished frame member without the use of a solid mandrel, die block or die button to support the wall or surface of the tube.